Terahertz time-domain spectroscopy ("THz-TDS") is a very powerful spectroscopic technique in the far-infrared spectral region. Terahertz radiation has been generated and detected using optically gated transmitters and receivers such as photoconducting dipole antennae as described in P. Smith et al., IEEE J. of Quantum Electronics, Vol, 24, No. 2, pp. 255-260 (1988) and N. Katzenellenbogen et al., Appl. Phys. Lett., Vol. 58, No. 3, pp. 222-224 (1991). With these techniques, terahertz spectroscopy offers a reasonably good signal-to-noise ratio (up to approximately 10.sup.4); it can be performed without special thermal stabilization apparatus such as cooled detectors; it can be realized in a compact system; and it offers a transmitter and detector technology which is compatible with integrated circuit technology.
Numerous experiments using terahertz time domain spectroscopy have been performed on solids, liquids, and gases. Some experiments have analyzed the spectrum of a terahertz signal affected by carriers in semiconductors and superconductors. Other experiments have performed terahertz time domain spectroscopy on water vapor as well as N.sub.2 O gas. Still other experiments have reported terahertz time domain spectroscopy of chemical compounds in the liquid phase. In all these experiments, the terahertz signal was transmitted through the object under study in a single illuminated volume region (usually 25 mm in diameter) to provide the spectral information about that homogeneous region.